1. Field of the Invention:
This invention relates to a shock absorber and, more particularly, to a shock absorber suited to be installed on a suspension system of a vehicle.
2. Description of the Prior Art:
Generally, a shock absorber includes a cylinder, a piston disposed movably in the cylinder and a piston rod connected with the piston projecting from the cylinder to the outside. The piston is provided with at least a port through which liquid flows when the piston rod is extended and at least a port through which liquid flows when the piston rod is contracted, and valve bodies are provided to open and close these ports.
The shock absorber installed on a suspension system of a vehicle provides preferably such a damping force property that the more rapidly the damping force rises, the lower the moving speed of the piston is, while it rises more slowly, the higher the moving speed of the piston is. Thus, the shock absorber has adopted such structure that liquid flows through at least an orifice to generate the damping force within a so-called orifice region when the piston speed is low, while the valve body is flexed by pressure of liquid flowing through the port to thereby generate the damping force within a so-called valve region when the piston speed is high.
Recently, such a shock absorber as disclosed, for example, in Japanese Utility Model Public Disclosure (KOKAI) No. 60-102536 has been proposed, which is capable of changing over the damping force into a plurality of stages respectively in the orifice region and valve region. The shock absorber includes a piston, a piston rod connected with the piston and a valve body, wherein a bypass path provided in the piston rod affords communication between two liquid chambers partitioned by the piston while the valve body is disposed in the bypass path. Any of a plurality of orifices or small holes having various bores provided in the piston rod or valve body is communicated t the bypass path by rotating the valve body for permitting liquid to flow therethrough, or the bypass path is interrupted completely. Thus, the flow rate of liquid flowing through the port in the piston is regulated.
According to the shock absorber disclosed in the Japanese Utility Model Public Disclosure as noted above, since the flow rate of liquid flowing through the bypass path is regulated by the orifices or small holes having various bores, a remarkable change in the damping force property within the orifice region is obtained by rotating the valve body. However, since the flow rate of liquid flowing through the bypass path or interrupted from flowing therethrough is small compared with that of liquid flowing through the port in the piston, only a slight change in the damping force property within the valve region is obtained by rotating the valve body. Thus, when the shock absorber is used for the suspension system of the vehicle, it is difficult to obtain the optimum ride comfort and controllability.